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    <h1>深入响应式原理</h1>
  

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    <p>现在是时候深入一下了！Vue 最独特的特性之一，是其非侵入性的响应式系统。数据模型仅仅是普通的 JavaScript 对象。而当你修改它们时，视图会进行更新。这使得状态管理非常简单直接，不过理解其工作原理同样重要，这样你可以避开一些常见的问题。在这个章节，我们将研究一下 Vue 响应式系统的底层的细节。</p>
<a class="dashAnchor" name="//dash_ref_如何追踪变化/Section/%E5%A6%82%E4%BD%95%E8%BF%BD%E8%B8%AA%E5%8F%98%E5%8C%96/0"></a><h2 id="如何追踪变化">如何追踪变化</h2><div class="vue-mastery"><a href="https://www.vuemastery.com/courses/advanced-components/build-a-reactivity-system" target="_blank" rel="sponsored noopener" title="Vue Reactivity">在 Vue Mastery 观看视频讲解</a></div>

<p>当你把一个普通的 JavaScript 对象传入 Vue 实例作为 <code>data</code> 选项，Vue 将遍历此对象所有的属性，并使用 <a href="https://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Reference/Global_Objects/Object/defineProperty" target="_blank" rel="noopener"><code>Object.defineProperty</code></a> 把这些属性全部转为 <a href="https://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Guide/Working_with_Objects#定义_getters_与_setters" target="_blank" rel="noopener">getter/setter</a>。<code>Object.defineProperty</code> 是 ES5 中一个无法 shim 的特性，这也就是 Vue 不支持 IE8 以及更低版本浏览器的原因。</p>
<p>这些 getter/setter 对用户来说是不可见的，但是在内部它们让 Vue 能够追踪依赖，在属性被访问和修改时通知变更。这里需要注意的是不同浏览器在控制台打印数据对象时对 getter/setter 的格式化并不同，所以建议安装 <a href="https://github.com/vuejs/vue-devtools" target="_blank" rel="noopener">vue-devtools</a> 来获取对检查数据更加友好的用户界面。</p>
<p>每个组件实例都对应一个 <strong>watcher</strong> 实例，它会在组件渲染的过程中把“接触”过的数据属性记录为依赖。之后当依赖项的 setter 触发时，会通知 watcher，从而使它关联的组件重新渲染。</p>
<p><img src="../../images/data.png" alt="data"></p>
<a class="dashAnchor" name="//dash_ref_检测变化的注意事项/Section/%E6%A3%80%E6%B5%8B%E5%8F%98%E5%8C%96%E7%9A%84%E6%B3%A8%E6%84%8F%E4%BA%8B%E9%A1%B9/0"></a><h2 id="检测变化的注意事项">检测变化的注意事项</h2><p>受现代 JavaScript 的限制 (而且 <code>Object.observe</code> 也已经被废弃)，Vue <strong>无法检测到对象属性的添加或删除</strong>。由于 Vue 会在初始化实例时对属性执行 getter/setter 转化，所以属性必须在 <code>data</code> 对象上存在才能让 Vue 将它转换为响应式的。例如：</p>
<pre><code class="hljs js"><span class="hljs-keyword">var</span> vm = <span class="hljs-keyword">new</span> Vue(&#123;
  <span class="hljs-attr">data</span>:&#123;
    <span class="hljs-attr">a</span>:<span class="hljs-number">1</span>
  &#125;
&#125;)

<span class="hljs-comment">// `vm.a` 是响应式的</span>

vm.b = <span class="hljs-number">2</span>
<span class="hljs-comment">// `vm.b` 是非响应式的</span></code></pre>
<p>对于已经创建的实例，Vue 不允许动态添加根级别的响应式属性。但是，可以使用 <code>Vue.set(object, propertyName, value)</code> 方法向嵌套对象添加响应式属性。例如，对于：</p>
<pre><code class="hljs js">Vue.set(vm.someObject, <span class="hljs-string">'b'</span>, <span class="hljs-number">2</span>)</code></pre>
<p>您还可以使用 <code>vm.$set</code> 实例方法，这也是全局 <code>Vue.set</code> 方法的别名：</p>
<pre><code class="hljs js"><span class="hljs-keyword">this</span>.$set(<span class="hljs-keyword">this</span>.someObject,<span class="hljs-string">'b'</span>,<span class="hljs-number">2</span>)</code></pre>
<p>有时你可能需要为已有对象赋值多个新属性，比如使用 <code>Object.assign()</code> 或 <code>_.extend()</code>。但是，这样添加到对象上的新属性不会触发更新。在这种情况下，你应该用原对象与要混合进去的对象的属性一起创建一个新的对象。</p>
<pre><code class="hljs js"><span class="hljs-comment">// 代替 `Object.assign(this.someObject, &#123; a: 1, b: 2 &#125;)`</span>
<span class="hljs-keyword">this</span>.someObject = <span class="hljs-built_in">Object</span>.assign(&#123;&#125;, <span class="hljs-keyword">this</span>.someObject, &#123; <span class="hljs-attr">a</span>: <span class="hljs-number">1</span>, <span class="hljs-attr">b</span>: <span class="hljs-number">2</span> &#125;)</code></pre>
<p>也有一些数组相关的注意事项，之前已经在<a href="list.html#注意事项">列表渲染</a>中讲过。</p>
<a class="dashAnchor" name="//dash_ref_声明响应式属性/Section/%E5%A3%B0%E6%98%8E%E5%93%8D%E5%BA%94%E5%BC%8F%E5%B1%9E%E6%80%A7/0"></a><h2 id="声明响应式属性">声明响应式属性</h2><p>由于 Vue 不允许动态添加根级响应式属性，所以你必须在初始化实例前声明所有根级响应式属性，哪怕只是一个空值：</p>
<pre><code class="hljs js"><span class="hljs-keyword">var</span> vm = <span class="hljs-keyword">new</span> Vue(&#123;
  <span class="hljs-attr">data</span>: &#123;
    <span class="hljs-comment">// 声明 message 为一个空值字符串</span>
    message: <span class="hljs-string">''</span>
  &#125;,
  <span class="hljs-attr">template</span>: <span class="hljs-string">'&lt;div&gt;&#123;&#123; message &#125;&#125;&lt;/div&gt;'</span>
&#125;)
<span class="hljs-comment">// 之后设置 `message`</span>
vm.message = <span class="hljs-string">'Hello!'</span></code></pre>
<p>如果你未在 <code>data</code> 选项中声明 <code>message</code>，Vue 将警告你渲染函数正在试图访问不存在的属性。</p>
<p>这样的限制在背后是有其技术原因的，它消除了在依赖项跟踪系统中的一类边界情况，也使 Vue 实例能更好地配合类型检查系统工作。但与此同时在代码可维护性方面也有一点重要的考虑：<code>data</code> 对象就像组件状态的结构 (schema)。提前声明所有的响应式属性，可以让组件代码在未来修改或给其他开发人员阅读时更易于理解。</p>
<a class="dashAnchor" name="//dash_ref_异步更新队列/Section/%E5%BC%82%E6%AD%A5%E6%9B%B4%E6%96%B0%E9%98%9F%E5%88%97/0"></a><h2 id="异步更新队列">异步更新队列</h2><p>可能你还没有注意到，Vue 在更新 DOM 时是<strong>异步</strong>执行的。只要侦听到数据变化，Vue 将开启一个队列，并缓冲在同一事件循环中发生的所有数据变更。如果同一个 watcher 被多次触发，只会被推入到队列中一次。这种在缓冲时去除重复数据对于避免不必要的计算和 DOM 操作是非常重要的。然后，在下一个的事件循环“tick”中，Vue 刷新队列并执行实际 (已去重的) 工作。Vue 在内部对异步队列尝试使用原生的 <code>Promise.then</code>、<code>MutationObserver</code> 和 <code>setImmediate</code>，如果执行环境不支持，则会采用 <code>setTimeout(fn, 0)</code> 代替。</p>
<p>例如，当你设置 <code>vm.someData = &#39;new value&#39;</code>，该组件不会立即重新渲染。当刷新队列时，组件会在下一个事件循环“tick”中更新。多数情况我们不需要关心这个过程，但是如果你想基于更新后的 DOM 状态来做点什么，这就可能会有些棘手。虽然 Vue.js 通常鼓励开发人员使用“数据驱动”的方式思考，避免直接接触 DOM，但是有时我们必须要这么做。为了在数据变化之后等待 Vue 完成更新 DOM，可以在数据变化之后立即使用 <code>Vue.nextTick(callback)</code>。这样回调函数将在 DOM 更新完成后被调用。例如：</p>
<pre><code class="hljs html"><span class="hljs-tag">&lt;<span class="hljs-name">div</span> <span class="hljs-attr">id</span>=<span class="hljs-string">"example"</span>&gt;</span>&#123;&#123;message&#125;&#125;<span class="hljs-tag">&lt;/<span class="hljs-name">div</span>&gt;</span></code></pre>
<pre><code class="hljs js"><span class="hljs-keyword">var</span> vm = <span class="hljs-keyword">new</span> Vue(&#123;
  <span class="hljs-attr">el</span>: <span class="hljs-string">'#example'</span>,
  <span class="hljs-attr">data</span>: &#123;
    <span class="hljs-attr">message</span>: <span class="hljs-string">'123'</span>
  &#125;
&#125;)
vm.message = <span class="hljs-string">'new message'</span> <span class="hljs-comment">// 更改数据</span>
vm.$el.textContent === <span class="hljs-string">'new message'</span> <span class="hljs-comment">// false</span>
Vue.nextTick(<span class="hljs-function"><span class="hljs-keyword">function</span> (<span class="hljs-params"></span>) </span>&#123;
  vm.$el.textContent === <span class="hljs-string">'new message'</span> <span class="hljs-comment">// true</span>
&#125;)</code></pre>
<p>在组件内使用 <code>vm.$nextTick()</code> 实例方法特别方便，因为它不需要全局 <code>Vue</code>，并且回调函数中的 <code>this</code> 将自动绑定到当前的 Vue 实例上：</p>
<pre><code class="hljs js">Vue.component(<span class="hljs-string">'example'</span>, &#123;
  <span class="hljs-attr">template</span>: <span class="hljs-string">'&lt;span&gt;&#123;&#123; message &#125;&#125;&lt;/span&gt;'</span>,
  <span class="hljs-attr">data</span>: <span class="hljs-function"><span class="hljs-keyword">function</span> (<span class="hljs-params"></span>) </span>&#123;
    <span class="hljs-keyword">return</span> &#123;
      <span class="hljs-attr">message</span>: <span class="hljs-string">'未更新'</span>
    &#125;
  &#125;,
  <span class="hljs-attr">methods</span>: &#123;
    <span class="hljs-attr">updateMessage</span>: <span class="hljs-function"><span class="hljs-keyword">function</span> (<span class="hljs-params"></span>) </span>&#123;
      <span class="hljs-keyword">this</span>.message = <span class="hljs-string">'已更新'</span>
      <span class="hljs-built_in">console</span>.log(<span class="hljs-keyword">this</span>.$el.textContent) <span class="hljs-comment">// =&gt; '未更新'</span>
      <span class="hljs-keyword">this</span>.$nextTick(<span class="hljs-function"><span class="hljs-keyword">function</span> (<span class="hljs-params"></span>) </span>&#123;
        <span class="hljs-built_in">console</span>.log(<span class="hljs-keyword">this</span>.$el.textContent) <span class="hljs-comment">// =&gt; '已更新'</span>
      &#125;)
    &#125;
  &#125;
&#125;)</code></pre>
<p>因为 <code>$nextTick()</code> 返回一个 <code>Promise</code> 对象，所以你可以使用新的 <a href="https://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Reference/Statements/async_function" target="_blank" rel="noopener">ES2017 async/await</a> 语法完成相同的事情：</p>
<pre><code class="hljs js">methods: &#123;
  <span class="hljs-attr">updateMessage</span>: <span class="hljs-keyword">async</span> <span class="hljs-function"><span class="hljs-keyword">function</span> (<span class="hljs-params"></span>) </span>&#123;
    <span class="hljs-keyword">this</span>.message = <span class="hljs-string">'已更新'</span>
    <span class="hljs-built_in">console</span>.log(<span class="hljs-keyword">this</span>.$el.textContent) <span class="hljs-comment">// =&gt; '未更新'</span>
    <span class="hljs-keyword">await</span> <span class="hljs-keyword">this</span>.$nextTick()
    <span class="hljs-built_in">console</span>.log(<span class="hljs-keyword">this</span>.$el.textContent) <span class="hljs-comment">// =&gt; '已更新'</span>
  &#125;
&#125;</code></pre>

  
  
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